The present invention relates to a circuit for the protection against overload of an electronic device which is fed from a source of dc voltage and is arranged at the starting or terminating of a connecting line over which communication signals are transmitted in the form of digital signals.
An "overloading" of the device within the meaning of the invention can result if too high a current flows for any reason. This may occur, for instance, in the event of a short circuit on the connecting line. A circuit is required for protection against overload upon every transmission by line of a signal when signals are digitalized or transmitted in pulse form. One known method of transmission is, for instance, the PCM2 method employed by the German Federal Post Office in which two spatially adjacent participants in the public communications network are together connected over a subscriber's line to a central office. The letters "PCM" stand for the well-known method of pulse code modulation by which communication signals are transmitted as digital signals. In the following this PCM2 method will be considered to be representative of all possible methods.
In the PCM2 method the analog signals are digitalized in the central office by suitable electronic circuits and sent out over the subscriber's line. At the end of the subscriber's line there is installed a device having an electronic circuit by which the incoming digital signals are converted back into analog signals. This is true also of the other direction. For the operation of the active elements of the electronic circuits there is provided in the central office a central current supply which supplies a dc voltage of, for instance, 95 V. For example, ten PCM devices which have these circuits and are all of the same construction are connected to this current supply, subscribers lines extending from each of them to, in each case, two subscribers. If a short-circuit occurs, for instance, on one of the devices, the supply of current can be interrupted, as a result of which not only the device concerned but also all the other devices drop out.
It is an object of the invention to provide a circuit for a system having a plurality of electronic devices connected to a common current supply by which assurance is had that in the case of an overload on a line which is connected to one device there will be no impairment of the other devices.